diff --git a/include/inja/function_storage.hpp b/include/inja/function_storage.hpp
index fc2dad2..b0091bd 100644
--- a/include/inja/function_storage.hpp
+++ b/include/inja/function_storage.hpp
@@ -71,6 +71,7 @@
   };
 
   struct FunctionData {
+    explicit FunctionData(const Operation &op, const CallbackFunction &cb = CallbackFunction{}) : operation(op), callback(cb) {}
     const Operation operation;
     const CallbackFunction callback;
   };
@@ -129,7 +130,7 @@
       }
     }
 
-    return { Operation::None };
+    return FunctionData { Operation::None };
   }
 };
 
diff --git a/single_include/inja/inja.hpp b/single_include/inja/inja.hpp
index 6710d81..26a6373 100644
--- a/single_include/inja/inja.hpp
+++ b/single_include/inja/inja.hpp
@@ -1589,6 +1589,7 @@
   };
 
   struct FunctionData {
+    explicit FunctionData(const Operation &op, const CallbackFunction &cb = CallbackFunction{}) : operation(op), callback(cb) {}
     const Operation operation;
     const CallbackFunction callback;
   };
@@ -1647,7 +1648,7 @@
       }
     }
 
-    return { Operation::None };
+    return FunctionData { Operation::None };
   }
 };